pip being a dependency is defined at upload time, it is parsed by the creator of the package, not the consumer, afaik if you used any other package manager, it

pip being a dependency is defined at upload time, it is parsed by the creator of the package, not the consumer, afaik if you used any other package manager, it would need to resolve dependencies from the additional info in the package index, resulting in pip being downloaded and installed before the desired package is installed